Sound introduced at the mouth propagates along the airways and through the parenchyma of the lung to be heard on the chest. The place where airway to parenchymal transition occurs appears to be in the small airways and is dependent upon small airway characteristics. We can find this point by timing measurements made during breathing of two gas mixtures. We will relate the location of this point to measures of small airway function in adult smokers and nonsmokers. By refining this technique we will be able to study children and infants. We will test normal children and children with well defined asthma, cystic fibrosis, and interstitial lung disease. We will measure the reflection of sound at the lung-chest wall interface. Since the amount of reflection is dependent upon the acoustic impedance of the parenchyma which is a function of density, estimates of lung water or location of hydro- or pneumothorax are possible. The development of these techniques may permit the noninvasive assessment and early detection of small airways or parenchymal disease in passive subjects of all ages.